The present invention relates to a vacuum variable capacitor used in a circuit of a large power oscillator, a high-frequency power-source circuit of a semiconductor equipment and a tank circuit of an induction heating apparatus.
Japanese Published Patent Application No. 7-78729 discloses a vacuum variable capacitor used in an impedance control device of various apparatuses such as a high-frequency power source of a semiconductor equipment or a high-frequency device of a large power oscillator. As shown in FIG. 4, this vacuum variable capacitor has a ceramic insulating cylinder 1 whose both openings are sealed by a fixed-side end plate 4 and a movable-side end plate 5 through connecting cylinders 2 and 3, respectively, so as to form a vacuum container 6. A fixed electrode 7 is provided by coaxially connecting a plurality of cylindrical electrode plates 7a, which are different in diameter with each other, to an inner surface of the fixed-side end plate 4. A movable electrode 9 is provided by coaxially connecting a plurality of cylindrical electrode plates 9a to a movable conductive member 8. The cylindrical electrode plates 9a are different with each other and from the cylindrical electrode plates 7a of the fixed electrode 7 in diameter so that the cylindrical electrode plates 9a can be inserted into and drawn from a space between the cylindrical electrode plates 7a with no contact therebetween.
The movable conductive member 8 comprises a hollow lead portion 8a which protrudes in the opposition direction of the movable electrode 8. An outer periphery of the hollow lead portion 8a is slidably supported by a cylindrical bearing 10 engaged and fixed to a center hole 5a of the movable-side end plate 5. A female thread 8b is formed at a closed end of the hollow lead portion 8a. A capacitance control screw 11 comprises a head portion 11a and a male thread 11b screwed with the female thread 8b. A screw receiving portion 12 is formed at an outer periphery of the hole 5a of the movable-side end plate 5. The capacitance control screw 11 is rotatably supported by the screw receiving portion 12 through a thrust bearing 13 for reducing a rotational torque of the capacitance control screw 11. By rotating the head portion 11a by means of a manual operation or motor drive, the movable conductive member 8 is vertically moved. According to this vertical movement, a total opposing area between the electrodes 7 and 9 is varied, and therefore the capacitance of the capacitor is varied. Accordingly, the impedance is controlled by this rotation of the head portion 11a. 
A cylindrical bellows 14 is made of soft metal and is disposed inside of the vacuum container 6. An end of the bellows 14 is connected to the movable-side end plate 5 and the bearing 10, and the other end of the bellows 14 is connected to the movable conductive member 8 so that the movable conductive member 8 and the movable electrode 9 can be vertically movable while keeping the sealing capacity of the vacuum container 6. Since the hollow lead portion 8a is insulated from the bearing 10 by mean of lubricating oil, an electricity flows through the hollow lead portion 8a to the bellows 14 in the vacuum container 6. That is, the bellows 14 functions as an electricity flowing path between an outer power source terminal (not shown) connected to the movable side end plate 5 and the movable conductive member 8. Accordingly, the high frequency current flows, for example, from the fixed-side end plate 4 to the movable-side end plate 5 through the capacitance between the movable conductive member 8 and the electrodes 7 and 9.
Lately, a load of a high frequency equipment has increased, and a vacuum variable capacitor for the equipment has become large to enable a large current flow according the increase of the high-frequency current. Therefore, a rotational torque of a capacitance control screw of the vacuum variable capacitor has also increased.